Name is bound multiple times is not reported in 'as' pattern

```
let f1 a a = ()
let f2 (a, b as c) c = ()
let f3 (a, b as c) a = ()
```

An error is expected in `f3`, but it's not reported:

<img width="226" alt="Screenshot 2021-07-23 at 12 42 41" src="https://user-images.githubusercontent.com/3923587/126764771-4f0e4b19-ed5b-4f4c-9948-09909cf7d396.png">
